The presence of vast world-wide coal reserves and the continuing need for stable supplies of liquid fuels suggest that coal-derived liquid fuels can play an important role as an energy source. This is particularly true in countries like the United States where transportation infrastructures are heavily oriented toward the transportation of liquid, rather than solid, fuels.
Under appropriate process conditions, coal liquefaction processes can supply a broad variety of liquid fuels ranging from heavy boiler fuels to gasolines. Additionally, many coal-derived liquids are useful as chemical feedstocks. For these reasons, liquid fuel producers and refiners continue to search for improved coal liquefaction processes as well as catalysts useful for improving the yield and quality of liquid product produced by these processes.
Early catalytic coal liquefaction processes such as the Bergius process tended to be complex multistep processes which were considered to be economically unfavorable. For example, the Bergius process required that coal was first mixed with a catalyst and then hydrogenated in a liquid phase in a slurry of heavy recycle oil. Liquid products were then distilled from the mixture and hydrogenated in a vapor phase over a solid catalyst. As noted by Nowacki in Coal Liquefaction Processes, Noyes Data Corp. 1979 page 19, principal disadvantages of this process included the need for high system pressures ranging up to 10,000 pounds as well as the need for the vapor phase hydrogenation.
Modern catalytic liquefaction processes have improved on the early Bergius process by reacting a slurry of coal and oil over a supported hydrogenation or hydrocracking catalyst in one or more stages of a multistage process. For example, U.S. Pat. No. 4,358,359 to Rosenthal discloses a two stage liquefaction process in which coal is first liquefied in a process-derived solvent and hydrogen in the absence of a catalyst. The coal and solvent mixture is then transferred to a hydrocracking reactor and hydrocracked in the presence of a supported hydrocracking catalyst. Other similar processes having a catalyst-free first liquefaction step and one or more subsequent supported catalyst hydrogenation or hydrocracking steps include U.S. Pat. Nos. 4,331,531, 4,317,446, 4,325,800, and 4,325,801.
While the processes noted above purportedly avoid many of the difficulties inherent in early liquefaction processes, such as the Bergius process, these processes are subject to other disadvantages inherent in many supported catalyst coal liquefaction systems. Most significantly, these type systems have been known to suffer from rapid deactivation of the supported catalysts, therefore requiring frequent catalyst regeneration and/or replacement or the use of upstream guard beds such as those disclosed in U.S. Pat. No. 4,325,800. Other disadvantages often associated with supported catalyst coal liquefaction processes include the agglomeration of small catalyst particles into larger, relatively less active catalyst species and poor conversion stemming from irregular catalyst dispersion within the liquefaction reactor. Additionally, such systems typically use ebullated bed reactors which are expensive to build and operate and must be operated under the narrow range of operating conditions required to provide proper ebullation of the coal, solvent and catalyst reaction mixture.
The solid catalyst-related problems noted above have led others to employ dispersed or soluble catalysts in coal liquefaction processes. Canadian Patent No. 1 249 536 discloses a single stage catalytic liquefaction process employing dihydrocarbyl-substituted dithiocarbamates of various metals as soluble catalyst precursors. The liquid catalyst precursor is converted to an active catalyst heating a mixture of coal, solvent and precursor as the mixture enters a reaction zone and reacts to produce a liquefied mixture. The liquified product is then fractionated by distillation or other means. As disclosed in that patent, the process produces relatively low liquid yields of less than about forty-two percent of the moisture free weight of the feed coal.
Others have attempted to minimize the difficulties associated with supported catalyst operation in coal liquefaction processes by operating a dispersed catalyst reactor upstream of a supported catalyst hydrocracking reactor. For example, U.S. Pat. No. 4,379,744 discloses the use of dispersed first stage liquefaction catalysts such as water-soluble salts of catalytic metals or oil soluble compounds containing catalytic metals such as napthenates of molybdenum, chromium or vanadium or phosphomolybdic acid. In this process, effluent from a first dispersed catalyst liquefaction step is transferred to a second process step in which the transferred effluent is hydrogenated in the presence of the hydrocracking catalyst. This process may be undesirable as it appears that carbon oxides evolved from coal dissolution are not removed prior to the hydrogenation step, thereby needlessly consuming hydrogen when these oxides are converted to methane.
While the dispersed catalyst liquefaction processes disclosed above suggest that dispersed catalysts may be useful in the first stage of a multistage coal liquefaction process, a need exists for improved dispersed catalyst processes. Such improved processes preferably should maximize yields of lighter, more valuable liquid products while minimizing the formation of heavier, less valuable products such as resid. It is also preferred that the processes avoid the needless hydrogenation of carbon dioxide. Additionally, it is preferred that the processes either avoid the use of supported catalysts or be carried out under conditions that minimize the supported catalyst problems discussed above.